My sister has moved to Italy
Lucky git!
.. and I need a grandma proof Skype to Skype only solution.
If using a wired handset (or cordless DECT telephone) is an option, then the most grandma-proof setup is probably an ATA plugged into her internet router, with the handset plugged into that. Then it will work just like a landline from her point of view.
I suggest you acquire and configure a suitable ATA and then send it to her fully set-up -- plug and talk at her end.
"ATA" = "Analog Telephone Adaptor" (eg. Linksys PAP2T series, etc..).
This will also require a voip service provider (unless you can find a Skype-locked ATA), but voip service is incredibly cheap ($2-5 monthly, or even a free service) so that shouldn't be an issue.